Bovine serum albumin and lysozyme adsorption on calcium phosphate particles

Two model proteins that are oppositely charged at neutral pH - bovine serum albumin (BSA) and lysozyme, with acidic and alkaline isoelectric points, respectively - are used to investigate the protein adsorption behaviour of hydroxyapatite and beta-tricalcium phosphate (βTCP) particles. Both calcium phosphate based particles are highly relevant for the fabrication of bioactive and resorbable bone implants. The investigations are carried out by combining zeta potential and Vis spectroscopy measurements. The changes of zeta potential and isoelectric point are determined as a function of added protein. Both proteins form a monolayer on βTCP, while on hydroxyapatite only semimonolayers were measured. For BSA, a side-on adsorption mode is suggested, whereas end-on adsorption appears to be most likely for lysozyme. The zeta potential curves as a function of adsorbed protein show that plateaus of the protein amounts adsorbed increase with charge saturation. In addition, the spatial charge distribution of both proteins is modelled to get a further understanding of the initial adsorption orientation of the biomolecules, supporting the findings from the experimental data. The reported findings can be transferred to the adsorption behaviour of a variety of proteins on calcium phosphate surfaces and are helpful for the fabrication of bone-analogous calcium phosphate/protein nanocomposites.